Alkaline storage batteries as sealed batteries have heretofore employed a pressure adjustment mechanism configured as follows. A sealing plate fixedly provided with a cap member is arranged at the open end of a bottomed cylindrical container case, and the open end edge of the container case is crimped through a gasket to close the upper part of the container case. An exhaust valve hole is provided, for example, at the central part of the sealing plate, the valve hole is airtightly sealed by a robber valve body arranged in a compressed state in a valve chamber surrounded by the sealing plate and the cap member during a normal period, and an internally accumulated gas is exhausted, by opening the valve hole when the gas pressure of the inside of the battery increases.
However, in the case where the temperature of the inside of the battery becomes an abnormally high temperature such as a temperature higher than 100° C. when rapid discharge occurs due to an external short circuit or when the battery is exposed under a high temperature, the rubber valve body is expanded, or the elastic modulus of the rubber valve body is changed. Thus, the pressure adjustment, mechanism may fail to operate, leading to an increase in pressure of the inside of the battery.
In the case of a nickel-metal hydride storage battery using a hydrogen-storage alloy for a negative electrode, when the temperature rises, the equilibrium pressure of the hydrogen-storage alloy which stores hydrogen increases, leading to a rapid increase in pressure of the inside of the battery. Thus, the pressure adjustment function may not be exhibited.
When the temperature of the inside of the battery becomes equal to or higher than 165° C., which is a melting point of a commonly-used resin-made separator, the resin-made separator may be melted, leading to an internal short-circuit.
Further, external components made of resin may be eroded as the surface temperature of the battery rises during rapid discharge due to an external short circuit or the like.
In relation to this, JP-A-9-237620 proposes that, by including 5 to 30% by weight of polypropylene in a rubber valve body a valve operating pressure at a high temperature can be reduced to cope with a change in internal pressure with a rapid rise in temperature of a battery. In JP-A-9-237620, it is intended that when an excessive pressure alter rapid temperature rise is released, 20 to 60% of the valve operating pressure is maintained at a high temperature, and a gas vent port is prevented from being opened. In such a rubber valve body, the amount of internally accumulated heat cannot be sufficiently released, and thus thermal runaway with a rapid temperature rise may occur. Therefore, the temperature of the battery may become higher to erode components existing outside the battery.
As described in JP-A-9-237620, natural rubber, SBR rubber and ethylene propylene rubber are proposed as elastic materials. Among them, ethylene propylene rubber is most excellent, but due to a time-dependent change in rubber elasticity, it is difficult to make settings for ensuring long-term reliability such as inhibition of leakage of an electrolyte solution.